Per Mertesacker has admitted that Arsenal were simply not good enough in their 3-2 Champions League defeat to Olympiacos.
The Premier League side twice got back into Tuesday's game after falling behind, but eventually failed to bounce back a third time as Alfred Finnbogason made the difference.
Mertesacker told the official Arsenal website: "We lacked that concentration and it is not understandable why we drifted and did not do the job we are used to doing. That is not acceptable and we will be punished [for that] in the Champions League.
"We need to get behind the ball and play better defensively. We lacked discipline, even when you get a goal back don't rush yourself. Get your discipline back and with possession you get your chances. If we are not disciplined and do not chase the ball we have got no chance.
"We have got a couple of days to look back on that performance. I think everyone will have regrets and it is good to take that away.
"It will take a couple of days to recover and to think about Manchester United and play in a different competition, which will be a good opportunity to come back to the Emirates and play our football with possession and dynamism."
